In the production of molded products such as plastic bottles, fluid plastic material is moved into the cavities of the molding machine and air is moved into the cavities of the molding machine at a predetermined pressure and velocity to urge flowable plastic into the spaces and shapes of the molding cavities of the molding machine.
Typically, a molding machine will have a plurality of duplicate mold cavities arranged in parallel so that a plurality of molded products will be formed simultaneously from the same molding machine on each cycle of operation of the molding machine. For example, six to twelve or even more mold cavities will simultaneously receive flowable plastic material and then receive compressed air that stretches the plastic in each of the cavities to help form a plurality of molded products. The plastic and pressurized air are received in surges during each cycle of operation of the molding machine.
It is desirable to deliver the pressurized air simultaneously to all of the mold cavities at the same and constant pressure during the cycles of operation of the molding machine. The compressors that supply pressurized air is moved through an inlet conduit to an air pressure regulator that reduces and controls the pressure of the air moving through a delivery conduit into the molding machine. The air pressure regulator typically includes a spring loaded regulator that reduces the pressure and modulates the flow of the air to supply the air at a constant lower pressure moving to the molding machine. When there is a change in the air pressure moving from the air pressure regulator through the delivery conduit and into the molding machine, the spring loaded diaphragm changes its valve position to adjust the flow of air approaching the molding machine, to compensate for the change.
In the typical prior art air supply system used with multiple cavity molding machines there is a likelihood that the pressure of the air delivered to the mold cavities will vary significantly during each cycle of operation of the molding machine due to deficiencies in the spring loaded air pressure regulator design and application. This regulator limits the pressure to all aspects of the machine and is located external to the machine for convenient access. When the blow valves open to supply air to the mold cavities, there is a delay in the regulator response due to its distance from the blow valves. This creates an undesirable reduction in pressure supplied to the molds. The prior art also has significant hysteresis, further slowing its response. The spring pressure varies with the amount of spring compression which also creates a delay in the regulator response. These types of regulators tend to have smaller orifice passageways resulting in higher pressure drop and slower response to downstream changes in pressures. These issues result in an excessive and undesirable decrease in the pressure of the air entering the mold cavities at the start of the air injection cycle. This excessive decrease in air pressure tends to lengthen the time to cycle the molding machine, to assure that the air pressure reaches the desired level and the plastic has been properly seated and cooled in the cavities of the molding machine. Also, these repetitive delays during each cycle of operation of the molding machine add up during the periods in which the molding equipment operates to produce the work products, so that fewer products are cast over a given period of time. This large initial decrease in pressure is often compensated for by elevating the regulator setpoint pressure. This results in wasting air as the application is over-pressurized above the optimum pressure. A dedicated secondary regulator could be installed closer to the blow valves to eliminate the delay, but that would increase costs and be less accessible and less convenient for the operator to adjust the pressure.
An example of the sequence of air pressures of a typical prior art device that supplies air to multiple cavity molding machines is shown in FIG. 1 of the drawings. FIG. 1 is a graph that shows how the air pressure supplied to a cavity of the molding machine is substantially below the desired pressure at the start of each cycle of the molding machine. This is illustrated by the drop of the air pressure from about 365 psig to about 229 psig. The typical prior art air supply device also does not achieve the setpoint pressure between cycles. This is illustrated by only achieving 348 psig instead of returning to 365 psig at the end of each cycle.
The prior art teaches that in order to minimize the amount of undesired reduction of the pressure of the air moving to the molding machine, a large air tank, known as an external “blow tank”, may be added to the air supply line that leads from the source of the regulated air to the molding machine. This is a result of the case where air that is delivered under pressure to the molding cavities is delivered to the mold cavities more rapidly than the oncoming replacement air moving into the air supply conduit, reducing the amount of pressure droop of the air entering the mold cavities. The external blow tank accumulates a large volume of the air at its desired controlled pressure. The blow tank is mounted externally due to space constraints inside the machine. However, this arrangement results in the regulator being mounted even farther from the blow valves, exacerbating the delayed response mentioned above. The blow tank also results in smaller changes in the pressure of the air during the operation of the molding machine. Although the blow tank tends to reduce the amount of pressure drop described above, the problem is not fully solved.
Even with the use of external blow tanks, the air pressure drop in the cavities of the molding machine is still not optimal with the prior art. The prior art air pressure regulator that reduces the pressure of the air moving from the compressor and through a blow tank on to the molding machine still cannot respond fast enough to detect the drop in air pressure at the molding machine and adjust the air flow to the molding machine in time to minimize the reduction in air pressure.